1. Field of the Invention
This invention relates to methods for producing salts of alkyl sulfoacetates and to alkyl sulfoacetate salt compositions. More specifically the invention relates to an improved method for preparing salts of alkyl sulfoacetates having reduced amounts of halogenated acetic acid. It also relates to sodium alkyl sulfoacetate compositions that are substantially free from monochloroacetic acid and its salts.
2. Description of the Related Arts
Sodium salts of alkyl sulfoacetates, in particular, lauryl sulfoacetate, have been recognized as being useful as surfactants in various dental care compositions. For example, sodium lauryl sulfoacetate was disclosed in U.S. Pat. No. 4,603,045 as a detergent for use in a toothpaste for cleaning natural teeth and composite tooth filling material. U.S. Pat. No. 4,807,649 discloses liquid denture cleansing compositions containing alkali metal or alkali earth metal salts of C.sub.10 -C.sub.16 alkyl sulfoacetates. U.S. Pat. No. 3,954,962 teaches oral hygeine products that may contain sodium lauryl sulfoacetate as a detergent. In addition, alkyl sulfoacetate salts may be used in bubble baths and synthetic soap products, especially in products intended for use by individuals sensitive to soap. Sodium lauryl sulfoacetate has been used at concentrations up to about 25% in luxury-type dry foam baths.
Because alkyl sulfoacetates are added to personal care products that typically come into contact with human mucousal membranes, these surfactants must be prepared or purified to contain minimal amounts of irritating byproducts.
When alkyl sulfoacetate surfactants are manufactured, irritating starting materials are present in the surfactant mixture. These byproducts are frequently undesirable and include sodium monochloroacetate and monochloroacetic acid. These materials, and especially sodium monochloroacetate, are sensitizing agents that are substantially irritating to mucous membranes. Thus, there is a need for such surfactants having minimal amounts of these byproducts for use in personal hygiene products.
No method is currently available for the preparation or purification of an alkyl sulfoacetate substantially free of starting alkyl alcohol, monochloroacetic acid or the corresponding metal salt.
A variety of solvents have been used in attempts to purify the alkyl sulfoacetate via extraction. Each attempt results in emulsions that are virtually inseparable. Purification methods relying on precipitation of the alkyl sulfoacetate result in sulfoacetate products containing appreciable amounts of monochloroacetate impurities. Further, purification using centrifugation of the sulfoacetate yields material having substantial amounts of monochloroacetate impurities.
Without being bound by a particular theory, it is believed that purification methods based on precipitation suffer from problems caused by "micelles" that exist in solution. It is believed that the micelles entrap contaminants present in the solution as the desired material precipitates or crystallizes.
Accordingly, methods for preparing alkyl sulfoacetates that are substantially free of monochloroacetic acid and/or its salts are needed.